1. Field of the Invention
The present invention relates to an optical coupler, and more particularly, to an optical coupler for coupling light emitted from an optical fiber or a laser diode with a slab waveguide such as a photonic crystal waveguide.
2. Description of the Related Art
Photonic crystals indicate artificial crystal structures in which dielectrics are periodically modulated. Generally, materials having a crystal structure affect the motion of electrons since a periodic potential occurs due to regular arrangement of atoms or molecules forming the materials. An important phenomenon occurring due to this is the formation of a band gap. This concept is also applied to photons. In this case, dielectrics act as potentials to photons. In this case, a bad gap occurs, which is distinguished from an electronic band gap and called as a photonic band gap.
Such photonic crystal structures may be applied to embody micro photonic devices, and more particularly, used as basic structures for optical waveguide of next-generation optical printed circuit board (O-PCB).
FIG. 1 is a schematic diagram illustrating a laser beam incident on a photonic crystal slab waveguide.
Referring to FIG. 1, in the case of silicon-on-insulator (SOI) now generally used as a photonic crystal slab waveguide, a vertical waveguiding structure consists of a silicon oxide layer 12, a silicon core layer 13, and an air layer, sequentially formed on a silicon substrate 11. Air holes h and line defects are formed in the silicon core layer 13, thereby obtaining the photonic crystal waveguide WG.
A laser beam is incident on the photonic crystal waveguide WG, and an optical fiber or a laser diode is generally used as a laser light source 100.
When light from the laser light source 100 is directly incident on the photonic crystal waveguide WG, there is shown much lower optical coupling efficiency than that of the case of a conventional dielectric waveguide.
The photonic crystal waveguide WG has a narrower width than other general waveguides. In detail, a diameter of light incident from an optical fiber or a laser diode is about 1.0 to 1.5 μm. On the other hand, since a width of the photonic crystal waveguide having the structure shown in FIG. 1 is generally about 300 nm, it is difficult to obtain high optical coupling efficiency by using general optical coupling technologies.
Due to the structural problems, a ratio of light outputted to the outside via an output terminal 14 of the photonic crystal waveguide WG is just 1 to 2% of the light incident from the laser light source 100.
As described above, when light emitted from an optical fiber or a laser diode is directly incident on a photonic crystal waveguide, optical coupling efficiency is just about 1 to 2%. Accordingly, there is required an optical coupler capable of improving optical coupling efficiency in the art.